Dielectric logging, introduced in the 1970's, added a new dimension to electromagnetic logging and has proven to be a valuable addition to formation evaluation. A new 1 GHZ, high-frequency dielectric logging tool has been developed with distinctive features relative to prior tools. The sensors are deployed by the tool using a pad on a flexible arm body instead of an in line configuration, thereby providing better sensor contact with the formation at the wall of the well borehole. Multiple antennas provide four independent measurements of formation dielectric properties with four different depths of investigation and four different vertical resolutions. Additionally, continuous operation of the transmitter and receiver channels, in addition to overcoming switching transients, also improves the ability to measure phase and amplitude of weak signals in highly attenuating (conductive) formations. A backup arm optionally deploys; a microlog pad to provide additional information on mudcake thickness, and to provide independent verification of thin bedding formation structure observed with the dielectric pad.
One of the dielectric measurements is derived using incident and reflected signals at the transmitter, independent of the antenna characteristics. In effect, this provides one set of dielectric properties at a zero source-to-detector spacing and hence maximizes vertical resolution of the logging tool. Internal calibration of the system over the dynamic range of the tool is made before and after logging, and is recorded in the calibration summary. Transformation of the calibrated phases and amplitudes to dielectric constant and resistivity (at the high frequency) is performed in realtime by the logging tool.
Log examples from several wells illustrate the improvements described above. Comparisons of conventional porosity logs with water filled porosity computed from the high frequency dielectric log are also made.
Dielectric tools determine the dielectric constant and conductivity of downhole formations from the real and imaginary parts of the complex propagation constant of electromagnetic waves travelling through the formations (T. J. Calvert, R. N. Rau and L. E. Wells, "Electromagnetic propagation . . . A new dimension in logging," presented at the Annual Meeting SPE, Bakersfield, Calif., April 1977, Paper 6542; D. S. Daev, Vysokochastonye Electromagnitnye Melody Issledevity. Skhvazhin:publ.House "Nedra," Moscow, 1970; R. A. Meador and P. T. Cox, "Dielectric constant logging, a salinity independent estimation of formation water volume," presented at the Annual Meeting SPE, Dallas, Tex., Oct. 1, 1975, Paper 5504). By measuring the phase difference and amplitude ratio between two points in the formation, the complex propagation constant is determined. Prior tools made this measurement differentially between the outputs of two receivers, while in the logging system disclosed, absolute phase and amplitude measurements are made at all receivers. Differential measurements, while reducing the effect of mudcake on the tool response, also reduce the dynamic range of the signals. To obtain high accuracy in the measurements, the receivers are optimally separated, with the separation limited, by the minimum detectable signal at the farthest receiver. Measurements of each receiver's phase and amplitude with respect to the transmitter increases the accuracy of the measurements by increasing the dynamic range of the signals without sacrificing signal strength. Absolute measurement also provides additional depths of investigation compared to differential measurements. If a differential measurement is desired, it can be obtained from the absolute measurement whereas the converse is not true.
Dielectric constants of downhole formations largely indicate the amount of water in the pores, since the dielectric constant of water is an order of magnitude greater than the highest dielectric constant of all other materials commonly found in the downhole environment. Water filled porosity can be determined from both the dielectric and the conductivity measurements (Poley, J.Ph., Nooteboom, J. J., de Waal, P. J.: "Use of VHF Dielectric Measurements for Borehole Formation Analysis," The Log Analyst vo. 19, pp 8-30 May-June 1978), and water saturation can be computed if formation porosity is known. Comparison of dielectric log results with water saturations obtained from other resistivity tools have provided means to probe the flushed zone of the formation. The apparent high frequency conductivity measured by conventional pad type resistivity devices. This is partly because the dielectric losses of water, which are large at high frequencies, cannot be separated from conductivity losses and partly because of mixing effects of pore distribution and fluid conductivity on electromagnetic fields.
This one GHz high frequency dielectric logging tool has been developed with distinctive features relative to prior tools, e.g., Calvert, supra. The high frequency dielectric tools has multiple antennas and greater spacing between the receivers to provide added and increased depths of investigation. The sensors of this disclosure are deployed on an independently articulated pad instead of being fixed on a mandrel body. A backup arm deploys a conventional microlog sensor. Unlike prior tools where the complex propagation constant measurement is made differentially between a pair of receivers, the high frequency dielectric measurements are made between the transmitter and each of several individual receivers. In addition to providing multiple depths of investigation and corresponding multiple vertical resolutions, this approach greatly increases the dynamic range of the signals measured. Unlike prior tools which multiplex the received data, the high frequency transmitter and receiver channels continuously sample the formation, thereby improving the signal-to-noise ratio or S/N of the measurement. This feature provides the ability to deploy a long spaced receiver with a deeper depth of investigation. Another novel feature of the present high frequency dielectric tool is that, for the first time, the measurement of both incident and reflected transmitter signals has been incorporated in a dielectric tool. Determination of dielectric constant and resistivity is made from the phase and amplitude measurements at each receiver and at the transmitter. The transmitter reflection measurement is equivalent to a receiver at zero spacing, maximizing the vertical resolution of the log, the tool has a Z axis oriented accelerometer to measure lengthwise acceleration to correct for erratic tool movement.
The mechanical design of the tool incorporates certain features such as a versatile pad mounting scheme, for the quick change, wear resistant sensor pad. The mechanical mandrel incorporates a fully independently actuating, dual pad linkage designed to operate a fully diverse range of borehole conditions. Each pad is carried by an independent parallelogram arm structure that delivers a constant pad force against the adjacent sidewall over the full range of pad displacement, thereby enabling the tool body to extend the sensors at any position in the borehole and still maintain proper pad contact with the borehole wall. With the extended reach of the arms, the tool is capable of logging in horizontal, deviated and washed out holes. The individual caliper measurement as well the borehole diameter are also recorded. The electrical power and communication between the instrument section and the dielectric sensor pad is furnished through a cable system consisting of a stainless steel cable encased in a braided, stainless steel flexible jacket. The coaxial cables are coiled at each pivot point of the parallelogram structure, enabling the cable to flex at each pivot point. Each metal cable is housed inside the arm assembly of the dielectric pad. The arm assembly with the cables can be disassembled as a single unit for quick field service. The dielectric sensor pad itself can be quickly changed. The pad has a hardened wear plate and mudcake plow assembly. The microlog pad is readily replaced with a metal shoe to log boreholes less than 77/8 inches in diameter. The tool is designed to be fully compatible with other tools positioned above and below it.
A block diagram of tool the present device utilizes the in phase and quadrature signal resulting from an 2 KHz square wave oscillator. The square wave signal is mixed with a one GHz signal to generate a transmitter signal pulsed at an audio frequency. Although this technique complicates the transmitter data handling, it improves measurement accuracy. This improvement occurs because the received signal is down converted to the audio frequency or 2 KHz by a mixer connected to the receiver antenna, and therefore the receiver is not required to contend with phase and amplitude changes of high frequency signals. The transmitter signal is amplified and fed to the transmitter antenna via a directional coupler which samples the incident and reflected signals for the measurement. The measurement is invariant to signal phase and amplitude changes that occur prior to arrival at the directional coupler. By design, the coupler is placed in close proximity to the transmitting antenna. The wide dynamic range of the receiver signals dictates the use of automatic gain control (AGC) amplifiers which reduce by approximately 40 dB the amplitude range seen by the phase selective detector in the receiver. The gain of these amplifiers is digitally controlled, and the gain control number is transmitted uphole through the data acquisition and telemetry system. The output from the AGC amplifiers sent to phase selective detectors where in phase and quadrature components of the 2 KHz signals are measured, digitized and transmitted to the surface.
Calibration of the tool is accomplished in two steps. An internal calibration of the receivers over the dynamic range of the tool is made before and after logging, and the results are stored and presented in the calibration summary. This calibration corrects for phase and amplitude deviations of the electronic circuits in the instrument section. The pad electronics, the sensors and the entire tool are calibrated in the second step, which involves using an external calibration medium placed over the sensors and on the pad. The external calibration is done prior to field use. During logging, the phase and amplitude from the transmitter incident and reflected signals, as well as the receiver signals, are acquired and recorded at a selected sample rate such as every 0.2 inches. The usual real time log display is usually based on an average of this data over selected intervals such as 0.25 feet. Based on algorithms developed from mathematically modeling a magnetic dipole, calibrated phases and amplitudes are transformed into the formation dielectric constant and 1 GHz resistivity. Normal and lateral resistivities from the microlog, and radii from the two caliper measurements are also presented at selected data intervals. The microlog measurement provides independent verification of mudcake thickness and bedding structure observed by the dielectric measurement.
An additional processing step, performed in realtime, is the computation of an apparent water filled porosity useful for a quick interpretation and a high frequency dielectric quality curve which is useful for assessment of log quality. Computation of apparent water filled porosity is a standard analysis technique for electromagnetic well logs. For dielectric well data determinations an additional level of sophistication is available in the analysis of apparent water filled porosity by using both the measured dielectric constant and high frequency resistivity together in the analysis. The additional information in dielectric logs may based to derive an indicator of log data quality.
Generally, this quick analysis technique computes a complex, apparent water filled porosity from dielectric logs and displays the imaginary part of the complex apparent water filled porosity as a quality indicator. The porosity computation may employ any mixing model which employs the measured complex dielectric constant in any way of its several forms and may be applied to any dielectric log. The procedure is most useful when the measured real and imaginary parts of the complex dielectric constant are of the same order of magnitude. While this procedure is valid for any complex mixing and model and for any dielectric log, the realtime implementation uses the complex refractive index model (R. N. Rau and R. P. Wharton, "Measurements of core electrical parameters at UHF and microwave frequencies," presented at annual Meeting SPE, Dallas, Tex., September 1980, Paper 9380; Shen, L. C., Manning, M. J. and Price, J. M., 1984, Application of Electromagnetic Propogation Tool in Formation Evaluation, Paper J: Transactions, SPWLA) and uses the described 1.0 GHz high frequency dielectric log tool. Setting S.sub.x0 equal to 1.0, the procedure solves the mixing equation for porosity, taking the real part of the computed porosity as the apparent water filled porosity anti displays the imaginary part of the computed result as a quality indicator.
These computations do not require input from other logging devices and may therefore be performed in real time or during post log analysis. When the dielectric log is run in combination with other devices such as density or neutron well logging tools, the apparent water filled porosity from the dielectric log may be overlaid with density or neutron porosity to provide a useful and quick analysis technique. The addition of the quality curve helps eliminate log intervals with poor data quality from the quick analysis.
Interpretation of the quality curve involves the identification of sharp spikes in the actuality curve, which are usually associated with rugosity effects on the log data, or the identification of extended intervals of smooth, non-zero values, which may represent any discrepancy in the model. Smooth, non zero values may be caused by lithologies, incorrect input of mud filtrate values, or flushed zone water saturation less than 1. Uses of the quality curve and apparent water filled porosity are illustrated in the log examples which follow.